Preheat oven to 425 degrees. Coat a baking pan with nonstick spray. Set aside.
In a large bowl, mix together egg, onions, cornmeal, taco seasonings, and salsa. Add beef or turkey and blend well.
Using a measuring tablespoon, scoop 1 level tablespoon for each meatball and roll into a ball. Repeat to make 48 meatballs. Place meatballs on a baking pan. Bake, uncovered, for about 12 minutes, until cooked completely. These can be prepped in advance and reheated before serving. They also freeze well.
To assemble each mini tostada: Place 1 tablespoon lettuce in each baked scoop. Drizzle with ½ teaspoon ranch or sour cream. Add 1 meatball. Drizzle ½ teaspoon salsa on top of each. Repeat to make 48 meatballs.
Serve at once.